1. Field of the Invention
The present invention relates to spindle-shaped magnetic goethite particles and spindle-shaped hematite particles, which have a uniform particle size, a larger minor axis diameter and a suitable aspect ratio and are very excellent in sintering preventing performance, and to spindle-shaped magnetic iron-based alloy particles which are obtained from said spindle-shaped hematite particles as a starting material, are excellent in dispersibility (high squareness and high orientation), excellent both in weatherability and in coercive force distribution and are preferably usable as household DAT, 8 mm, Hi-8 tapes, VTR tapes for business purposes, computer tapes or floppy disks.
2. Description of the Prior Art
Miniaturized and lightweight magnetic recording and reproducing devices for long-time recording in audios, videos and computers such as household DAT, 8-mm video tapes, Hi-8 tapes, VTR tapes for business purposes, computer tapes or floppy disks have recently shown a remarkable progress. Especially, video tape recorders (VTR) have rapidly spread widely and the development of miniaturized and lighter-weight VTR for longer-time recording, particularly attempted shifting from analogue to digital systems, has been rapid. With this development, magnetic recording media such as a magnetic tape have been strongly required to have a higher performance and a higher recording density and to improve reliability in recording.
Magnetic recording media are required to improve higher picture qualities, higher output characteristics and especially frequency characteristics, and for this purpose, it is necessary to improve the residual flux density (Br), the coercive force, the dispersibility, the packing property, the surface smoothness of a magnetic tape and the S/N ratio.
These properties of magnetic recording media have a close relationship with the magnetic particles used for the magnetic recording media. In recent years, magnetic iron-based alloy particles have attracted attention due to their coercive force and saturation magnetization which are superior to those of conventional iron oxide magnetic particles, and have been put to practical use as magnetic recording media such as digital audio tapes (DAT), 8-mm video tapes, Hi-8 tapes, VTR tapes for business purposes, computer tapes and floppy disks. Such magnetic iron-based alloy particles, however, are also strongly demanded to improve the properties.
The properties of magnetic recording media are described in detail as follows: In order to achieve high picture qualities as video magnetic recording media, it is required to improve SIN ratio and video frequency characteristics. For this improvement, it is important to improve the dispersibility of magnetic particles in a coating material, orientation in a coating film and the packing property and to improve the surface smoothness of the magnetic recording media. In order to improve the video frequency characteristics, it is necessary that the coercive force of the magnetic recording media is not only high, but the residual flux density is large, and further that the S.F.D. (Switching Field Distribution) of the magnetic recording media, namely, the coercive force distribution is small. In addition, the repeated running ability, still characteristics or reliability of the magnetic recording media in recording for use in a severe environment should also be secured, in other words, the improvement of durability is also important.
It is considered that such metal magnetic particles preferably have a larger particle size for the dispersibility and weatherability thereof, and preferably have a larger aspect ratio for the squareness and orientation thereof in the form of a coating film. On the other hand, those having a smaller particle size are preferable from the viewpoint of surface smoothness and noises, but as the particle size is decreased, the particles are hardly dispersed and their weatherability is deteriorated. From the viewpoint of saturation magnetization, those having a larger particle size with an excellent distribution of particle sizes are preferable, but if the particle size is larger than necessary, the coercive force tends to be reduced, and it is thus necessary to maintain the coercive force by increasing the aspect ratio of the metal magnetic particles.
Generally, for preparation of the metal magnetic particles, goethite particles as the starting material, hematite particles obtained by heat dehydrating the goethite particles, or particles prepared by adding heterogeneous non-iron metals to these particles are heat-treated, if necessary, in a non-reducing atmosphere and then heat-reduced in a reducing atmosphere to form metal magnetic particles. During this process, it is necessary to suitably regulate the shape and size of goethite particles as the starting material and to prevent the fusion of particles upon heat treatment such as heating and reduction or the deformation and destruction of a single particle, whereby the shape and size of goethite particles as the starting material are maintained and inherited by the resulting metal magnetic particles.
Morphologically, there are two kinds of goethite particles as the starting material, that is, needle-shaped goethite particles obtained from an alkali hydroxide as a base and spindle-shaped goethite particles obtained from an alkali carbonate as a base. Although the needle-shape goethite particles are characterized in that generally those having a larger aspect ratio can be easily obtained, there is the problem that they are inferior to spindle-shaped particles in particle size distribution and those having a small particle size are hardly obtained. This particle size distribution is an indicator of the uniformity of primary particles and is thus also closely related to the coercive force distribution of the metal magnetic particles or to their weatherability, that is, oxidation stability. Japanese Patent Application Laid-Open (KOKAI) No. 5-98321 discloses the technique of obtaining fine needle-shaped particles having a relatively large aspect ratio, a high coercive force and a large saturation magnetization, but the oxidation stability and heat resistance of the metal magnetic particles or the weatherability of magnetic coating film therefrom have not sufficiently been examined.
On the other hand, the spindle-shaped goethite particles are characterized by being generally superior in particle size distribution of, but those having a larger aspect ratio are hardly obtained, and if the particle size is increased, the coercive force of the metal magnetic particles is low as compared with the needle-shaped particles, and thus the coercive force is maintained usually by decreasing the particle size. As a result, since the particle size is relatively small, there is the problem that the dispersibility thereof in a coating material is not good, and further due to the low aspect ratio, the squareness and orientation of the coating film are low, and the weatherability of the coating film is not satisfactory due to the small particle size in spite of good particle size distribution. Japanese Patent Application Laid-Open (KOKAI) No. 5-62166 discloses the technique of securing the coercive force and improving the dispersibility on the basis of the idea of enlarging the aspect ratio of spindle-shaped metal magnetic particles, but the oxidation stability of the metal magnetic particles and the weatherability of magnetic coating film therefrom are not taken into consideration.
For the reason described above, metal magnetic particles used in media for magnetic recording and reproducing devices in audios, videos and computers such as household DAT, 8-mm video tapes, Hi-8 tapes, VTR tapes for business purposed, computer tapes or floppy disks generally make use of needle-shaped metal magnetic particles with a coercive force of 103.5 to 143.2 KA/m (1300 to 1800 Oe) having a large particle size, a large aspect ratio, and a high squareness and orientation thereof in a magnetic coating film. However, these particles are not satisfactory in respect of the particle size distribution as described above, and an attempt of improving this feature is being conducted, but because of a relatively broad particle size distribution as compared with that of spindle-shaped particles, the weatherability thereof cannot be satisfactory regardless of their large particle sizes.
As the heat reduction apparatus used in the heat reduction step, there are known a fluidized-bed reduction apparatus for heat reducing the starting material while fluidizing it in a powdery forms a fixed-bed reduction apparatus for heat reducing the starting material after granulating and forming it into a fixed bed, and a transfer-bed reduction apparatus for transferring a bed having a fixed bed formed therein.
In raising a demand for mass-production technology accompanied with an increasing demand for metal magnetic particles, an apparatus (including a transfer bed) provided with a fixed bed enabling mass-production which are free of particle scattering regardless of a larger flow of a reducing gas such as hydrogen, etc. is industrially and economically advantageous.
However, if heat reduction is conducted under a hydrogen gas atmosphere by forming a fixed bed, the partial pressure of steam is increased by rapid reduction in a lower part of the fixed bed, and the destruction of the shape of particles and the excessive growth of the minor axis occur in an upper part of the bed as compared with the lower part of the bed, thus easily causing a difference in the characteristics of particles between the lower and upper parts of the bed, and it is difficult to obtain metal magnetic particles having uniform characteristics.
Generally, it is necessary that the fusion of particles as the starting material or the deformation or destruction of a single particle is prevented so that the shape and size of the goethite particles as the starting material or hematite particles are maintained and inherited by the resulting metal magnetic particles. The metal magnetic particles whose shape was destroyed cannot achieve a high coercive force due to a reduction in shape anisotropy and the particle size distribution is lowered. Further, even in production of magnetic recording media, due to an increase in the force between particles in the step of kneading and dispersing them in a binder resin or due to an increase in the magnetic cohesion, the dispersibility of the particles is lowered and the squareness thereof in a magnetic coating film is lowered, and thus magnetic recording media having excellent SFD cannot be obtained.
Accordingly, there is a strong demand for a heat reduction method by which the destruction of particle shape is prevented as much as possible and simultaneously the characteristics of metal magnetic particles in lower and upper parts of a fixed bed are made uniform.
As the method of obtaining magnetic iron-based alloy particles having uniform characteristics and high coercive force by forming a fixed bed, there are known a method in which needle-shaped magnetite is obtained by reduction at a temperature of less than 350xc2x0 C. while specific steam is introduced at an initial stage of reduction, followed by reduction in the temperature range of 350 to 550xc2x0 C. (Japanese Patent Application Laid-Open (KOKAI) No. 4-224609), a method of heat reduction in which a gas superficial velocity of a hydrogen gas is adjusted in a specific range (Japanese Patent Application Laid-Open (KOKAI) No. 54-62915) and a method in which a material to be reduced is transferred continuously to a belt capable of passing a gas and provided in a gas passaging reactor, and the material is heat-reduced while a hydrogen gas is passed in a vertical direction (Japanese Patent Application Laid-Open (KOKAI) No. 6-93312).
Under the background described above, it is required to produce metal magnetic particles having uniform characteristics by use of an apparatus having a fixed bed formed therein, which are spindle-shaped particles being excellent in dispersibility (high squareness, high orientation) and excellent both in weatherability and in coercive force distribution and having a coercive force of preferably 103.5 to 143.2 KA/m (1300 to 1800 Oe).
Conventionally, with respect to the spindle-shaped goethite and spindle-shaped metal magnetic particles, Japanese Patent Publication (KOKOKU) No. 1-18961 discloses the technique in which the desired coercive force is obtained by selecting the particle size and aspect ratio suitably and the specific surface area is reduced to decrease the viscosity thereof in a coating material, but the oxidation stability of the metal magnetic particles and the squareness and orientation thereof in a coating film are not taken into consideration.
With the idea of increasing the aspect ratio as is the case with the conventional needle-shaped metal magnetic particles, Japanese Patent Application Laid-Open (KOKAI) Nos. 9-295814 and 10-245233 disclose the techniques of achieving a high coercive force and an excellent coercive force distribution in the spindle-shaped metal magnetic particles, but the oxidation stability is not taken into consideration. Japanese Patent Application Laid-Open (KOKAI) No. 10-245233 supra describes that the coercive force distribution thereof in a coating film is excellent where the relationship between the crystallite sizes D104 and D110 of spindle-shaped hematite particles is in a specific range, but the relationship with the crystallite size of goethite particles as the starting material is not referred to, and this prior art is unsatisfactory in respect of the sintering of particles in the heat treatment step or in the destruction of the shape of particles.
Japanese Patent Application Laid-Open (KOKAI) Nos. 7-126704, 8-165501 and 8-165117 disclose the techniques of obtaining fine spindle-shaped metal magnetic particles with a high coercive force containing Co and Al, but the oxidation stability of the metal magnetic particles is not sufficiently examined in Japanese Patent Application Laid-Open (KOKAI) No. 7-126704 and the level in Japanese Patent Application Laid-Open (KOKAI) No. 8-165501 is not satisfactory. In Japanese Patent Application Laid-Open (KOKAI) No. 8-165117, the crystallite size ratio D020/D110 of spindle-shaped goethite particles is specified, but the growth for formation of surface layer particles from seed crystal particles is not mentioned.
With respect to the heat resistance of metal magnetic particles, Japanese Patent Application Laid-Open (KOKAI) No. 59-207024 discloses metal magnetic particles whose differential thermal curve is not changed until 80xc2x0 C., and describes those containing 7 atom % of Al with an ignition temperature of 130xc2x0 C. though their shape is not revealed. Similarly, in Japanese Patent Application Laid-Open (KOKAI) No. 2-19161, even those having a high ignition temperature of 121xc2x0 C. at highest and their heat resistance is not satisfactory.
Japanese Patent Application Laid-Open (KOKAI) No. 10-334455 discloses the technique of obtaining magnetic recording media having excellent head sliding characteristics and good storage properties by adjusting the contents of Co, Al and rare earth elements in metal magnetic particles within a specific range, but the particle size, shape, and particle size distribution of goethite particles as the starting material are not examined, and the coercive force, weatherability and dispersibility are not sufficiently examined.
For the spindle-shaped goethite particles described above, while maintaining their excellent particle size distribution, including the above problem, that is, maintaining the coercive force in a state where the particle size is enlarged, the enlargement of the aspect ratio is examined on the basis of the same idea as for needle-shaped metal magnetic particles in order to obtain a high squareness and high orientation of the spindle-shaped particles, but satisfactory particles have not been obtained yet, and it cannot be said that the effect or influence on individual characteristics are sufficiently examined.
In the method described in Japanese Patent Application Laid-Open (KOKAI) No. 4-224609 supra, the atmosphere for heating is hydrogen, but the heating rate is not specified, and it cannot be said that oxidation stability etc. are sufficiently examined.
Further, in the method described in Japanese Patent Application Laid-Open (KOKAI) No. 54-62915 supra, the coercive force of the resulting metal magnetic particle powder is as low as 95.5 KA/m (1200 Oe), probably because the atmosphere for heating is nitrogen and the superficial velocity for a reducing gas is low, and further it cannot be said that the dispersibility thereof in a coating material and the squareness and orientation thereof in a coating film are sufficiently examined.
Further, in the method described in Japanese Patent Application Laid-Open (KOKAI) No. 6-93312 supra, the magnetic particles does not contain Co, and it cannot be said that the oxidation stability of the magnetic particles, the dispersibility thereof in a coating material and the squareness and orientation thereof in a coating film are sufficiently examined.
An object of the present invention is to provide metal magnetic particles having a high coercive force of e.g. 103.5 to 143.2 KA/m (1300 to 1800 Oe), which are spindle-shaped, and excellent in dispersibility (high squareness, high orientation) and has both excellent weatherability and coercive force distribution.
Another object of the present invention is to provide metal magnetic particles having uniform characteristics in lower and upper parts of a formed fixed bed.
Still other objects and advantages of the present invention will be evident to those skilled in the art from the following description.
As a result of their extensive study on obtaining spindle-shaped metal magnetic particles characterized by excellent particle size distribution, which are further endowed with the characteristics of needle-shaped metal magnetic particles, that is, the high dispersibility, high squareness and high orientation thereof in a coating film and with further excellent particle size distribution, the present inventors achieved these objects by an idea completely different from the conventional idea and succeeded in providing metal magnetic particles with further improved above-described characteristics in a bed comprising a fixed bed formed therein, thereby arriving at the present invention.